MSL is the latest in the organization’s nearly four decade long robotic presence on the red planet. Its payload, the rover Curiosity, will explore and tell us more about our cosmic neighbor. But Curiosity isn’t the most interesting part of the mission. It’s how the rover will land that’s spectacular. A lander won’t deliver Curiosity to the Martian surface; a descent module called the Sky Crane will lower the rover from a tether. Curiosity will touch down directly with its wheels, ready to rove. Risky? Perhaps. Ingenious? Absolutely. It’s also crucial that the Sky Crane work perfectly. A failure of the system could set the next decade of planned Martian exploration back years.

Challenges of Landing on Mars

Rover Sojourner on the surface of Mars. Credit: NASA/courtesy of nasaimages.org.

Landing on Mars isn’t easy; it’s practically a robot graveyard. Two thirds of all landers sent there have crashed or lost contact with Earth, and half of the missions launched haven’t made it to the planet at all. Successful entry, descent, and landing - or EDL - on Mars poses a real challenge for engineers.

Mars’ whole surface area is only about as large as the Earth’s land masses put together. Its small size gives the planet a weaker gravitational pull, only about one-third that felt on Earth. Mars does have an atmosphere. It’s composed largely of carbon dioxide with traces of nitrogen and argon and is only one percent as thick as Earth’s.

Mars’ thin atmosphere makes landings more difficult, much more so than landing on a body with no atmosphere like our Moon. As a spacecraft falls through Mars’ thin atmosphere, it generates friction and heat, but not enough to significantly slow the spacecraft. The thin atmosphere also means there are fewer air molecules to inflate a parachute, rendering yet another braking method only moderately effective.

The time delay in communications between engineers on Earth and their spacecraft on Mars is another challenge. Radio signals take roughly 20 minutes to travel one way between the two planets. A spacecraft only has six minutes between first coming into contact with the Martian atmosphere and reaching the surface, so real-time adjustments to a landing sequence are out of the question. Whatever EDL engineers devise, it has six minutes to work autonomously.

As a final hurdle, there is no way to simulate a Martian landing on Earth. There’s no way to test a lander in one-third gravity, no easy way to test a parachute in one percent atmosphere, and only so much is known about the Martian topography as far as landing hazards. EDL technology goes through its full profile once, and it has to work.

Lucky for NASA, it found an EDL system that worked in its early years of Martian exploration.

How NASA lands on Mars

The Viking lander. Credit: NASA/courtesy of nasaimages.org.

NASA’s Jet Propulsion Laboratory (JPL) began working on the Mars problem in 1968 when President Johnson called for a landing on the red planet to mark the United States’ bicentennial. At the time, NASA was enjoying its Apollo-inflated budget and was able to devote significant funds to developing and testing a Martian EDL system. What emerged was the Viking program - twin orbiters and landers would go to Mars to increase the odds of a successful landing.

The Viking 1 lander spent its interplanetary journey stored inside a protective casing. Inside, the lander was sandwiched between a back shell that housed a parachute on top and an ablative heat shield on the bottom. The spacecraft arrived at Mars and entered orbit on July 19, 1976 before firing its thrusters to enter the Martian atmosphere on July 20.

The heat shield burned away with the heat produced by friction with the Martian atmosphere, slowing the lander’s descent. Four miles above the surface, the back shell released and a 52-foot wide parachute deployed. It unfurled and inflated, stabilizing and further slowing Viking 1’s descent as the heat shield, now spent, was jettisoned. A landing radar on the lander’s underside sprung to life, determining the rest of the EDL sequence. A little under a mile above the surface, the lander fell away from the back shell and parachute, triggering ignition of retrorockets on its underside. Under its own power, the landing slowed to a descent rate of just 7 feet per second. Sensors in the legs triggered shut down of the retros on contact with the surface. Viking 1 settled happily onto Mars. Viking 2 followed suit on September 3, 1976. This formula successfully delivered the Mars Phoenix Lander to the surface in 2008.

Spirit, packed and ready to go. Credit: NASA/courtesy of nasaimages.org.

The same entry and descent sequence delivered the rovers Sojourner, Spirit and Opportunity to the surface in 1997 and 2004 respectively. But the landing was a little different. All rovers have arrived at Mars housed in airbags. Developed for the Sojourner rover, payload of the Mars Pathfinder Mission, the design was a way to hit a wider target area at faster speeds while spending less money.

Airbags would allow the lander to free-fall through its final descent - doing away with heavy and finicky retrorockets - and bounce along the surface before coming to its final resting point - eliminating the need for expensive and precise landing instruments.

Sojourner was stored in a pyramid-shaped case. After the heat shield burned away in the upper atmosphere and fell away, between four and six miles from the surface, the pyramid fell from the back shell on a 65-foot long tether. The airbags inflated explosively from its sides at the same time that retrorockets on the back shell ignited. The retros slowed the payload further before cutting the bridle and letting it fall the rest of the way to the surface.

The airbags used to deliver the MER rovers. Shown inflated in JPL's test bed where engineers studied how they would deflate. Credit: NASA/courtesy of nasaimages.org.

The airbags bounced and rolled along the surface before coming to a stop. When onboard sensors registered no movement, the airbags deflated and the pyramid’s panels unfolded like a flower’s petals. One opened first to right the rover in case in landed on one of its sides. The panels opened to reveal a healthy Sojourner, ready to drive down one of the ramps created by the panels and explore its new home.

The same EDL delivered the Mars Exploration Rovers Spirit and Opportunity to the surface in 2004, but the rovers pushed the system to its limits. Sojourner is about the size of a microwave and weighs a little over 23 pounds. Spirit and Opportunity dwarf their predecessor weighing in at about 408 pounds each. The MER rovers’ size was dictated by what could fit inside the pyramid casing, and the weight the airbags could safely carry across a potentially ragged landing area.

The airbag landing system isn’t an option for Curiosity. Weighing in at an impressive 1,654 pounds (that’s three-quarters of a ton), it is the size of an SUV. Airbags large enough to cushion its landing would be too heavy for the Atlas launch vehicle and likely fail under the weight of the air required to inflate them.

Similarly, a larger parachute to slow Curiosity’s initial descent - or to attempt a full parachute-controlled landing - was out of the question. A parachute large enough would be too heavy for the launch vehicle and unlikely to fully inflate before its payload reached the Martian surface. A Viking-style landing for the rover was also a poor option. Not only do retrorockets add substantial weight for the rover to then carry around the surface, but rockets protruding on its underside also create a potential sticking hazard.

The Jet Propulsion Laboratory was also keen to move away from the inevitability of driving a rover off a landing platform as it had with the previous rovers. It was a delicate maneuver with the constant possibility that the ramps would be stuck against a rock. Getting to Mars and not being able to drive a healthy rover off a lander would be a poor end to a mission.

It was clear that for NASA to deliver the larger and heavier payload with less room for failures, it would need a new landing system. Enter the Sky Crane.

Curiosity will begin its entry and descent like its predecessors: wheeled legs folded and stored between a back shell and parachute on top and ablative heat shield on the bottom.

Four miles above the surface, the Martian atmosphere will have provided enough resistance against the rounded heat shield to slow Curiosity to 1,000 miles per hour, about Mach 2 in that atmosphere. Burned away, the heat shield will jettison and the parachute will deploy. At 65 feet in diameter, it is the largest ever sent to Mars. In a little over three vertical miles, the parachute will slow Curiosity from 1,000 to a relatively calm 187 miles per hour before it is jettisoned with the back shell.

The parachute’s release will also trigger the jettison of the heat shield, exposing Curiosity’s underside and uncovering a ground sensing radar that will determine the rest of the landing sequence.

A little more than half a mile above the surface, the descent vehicle’s retro rockets will ignite and explosive bolts will separate it from the back shell. These retrorockets will take control of Curiosity’s descent. When the radar senses that it is about 115 feet above the surface, the descent vehicle will release Curiosity on a 65 foot tether. It will slow its descent to less than one mile per hour as it lowers the dangling rover to the surface.

Once Curiosity’s wheels touch down, explosive bolts will fire to sever the tether from the rover. The descent vehicle’s retros will carry it away to crash a safe distance from the rover. Curiosity will be free to begin its exploration of Mars.

What Happens if it Fails?

The Sky Crane lowers Curiosity.Credit: NASA/JPL-Caltech

There are a lot of moving parts on this system and a lot of things that have to happen at precisely the right time. It’s unlikely but still possible that a flaw in the parachute or heat shield could end the mission early with a crash landing. More problematic is the landing radar.

Any false information could end with Curiosity falling from an unrecoverable height, breaking its wheels and remaining in one place for its operable lifetime. Likewise if the tether snaps when Curiosity is released. If the explosive bolts connecting Curiosity to the descent module fail once Curiosity is safely on the surface, the descent vehicle could unwittingly pull the rover back off its wheels and drag it to a crash landing.

These scenarios are possible but improbable. Like every system JPL and NASA builds, the Sky Crane is designed to work perfectly on Mars. But if the Sky Crane fails, MSL and Curiosity won’t be the only casualties. In cooperation with the European Space Agency (ESA), NASA has an ambitious ten year plan for exploring Mars, much of which depends on the Sky Crane.

Curiosity’s landing on Mars in the summer of 2012 is only the first step. In 2014, an orbiter will arrive at Mars to measure the escape rate of the Martian atmosphere. This will give scientists the data needed to reverse-engineer Mars’ atmosphere, determine its early composition, and figure out when in Mars’ history the planet was likely to harbor life.

NASA and the ESA will join together for the 2016 and 2018 ExoMars missions. In 2016, NASA will launch the ESA’s orbiter and an Entry, Descent, and Landing Module (EDM). The orbiter will use onboard instruments to detect and study trace gases in the atmosphere while the EDM will use sensors to evaluate a landed payload’s EDL performance and study the landing site. In 2018, NASA will send two rovers to Mars – one American and one European. Both will land together at the same site carrying different payloads. The ESA rover will carry a drill and a suite of instruments for exobiology and geochemistry research.

The NASA rover, delivered by the Sky Crane, will be the first step in NASA’s sample return mission. It will travel to known interesting areas and collect samples, each roughly the volume of a ballpoint pen, and store them. A second lander carrying a “fetch” rover will follow the sample collection rover and will also arrive on Mars by the Sky Crane. It will collect the samples and transfer them to an ascent vehicle that will launch into Martian orbit. A third mission will launch to collect the orbiting sample and return them to Earth.

The potential payoff of a sample return mission is exciting. Having Martian samples on Earth, particularly ones that didn’t fall unprotected through our atmosphere, would give scientists access to Mars in a whole new way. Samples would be on hand as new tests develop and technologies emerge. Subsequent missions to Mars could be much more specifically targeted, revealing a wealth of knowledge not only about the red planet, but about the history of our solar system.

The views expressed are those of the author and are not necessarily those of Scientific American.

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